1. Field of the Invention
The present invention relates to a dielectric filter using transverse electromagnetic (TEM) mode and constructed by forming a plurality of inner conductors in a dielectric block and by forming an outer conductor on outer surfaces of the dielectric block and, more particularly, to a dielectric filter of this construction having an improved spurious response characteristic.
2. Description of the Related Art
FIG. 8 shows the structure of a conventional dielectric filter using TEM mode. In FIG. 8 and similar illustrations referred to below, a dotted area represents an exposed portion of a dielectric block (non-conductor-formation portion).
In this dielectric filter, as shown in FIG. 8, two resonator holes 2a and 2b are formed through a dielectric block 1 in the form of a rectangular prism so as to have openings in a pair of opposite end surfaces of the dielectric block 1. An inner conductor 3 which functions as a resonating conductor is formed on the inner cylindrical surface of each of the resonators 2a and 2b. An outer conductor 4 which functions as a ground conductor is formed generally over all the outer surfaces of the dielectric block 1. A pair of input/output electrodes 5 are formed in predetermined portions of the outer conductor 4. Each of the resonator holes 2a and 2b has an inner conductor non-formation section 3a formed in the vicinity of one opening end surface of the resonator hole to separate (maintain an open circuit) between the inner conductor 3 and the outer conductor 4. At the other opening end of the resonator hole (at the rear side as viewed in FIG. 8), the inner conductor 3 is electrically connected (shorted) to the outer conductor 4. The input/output electrodes 5 are externally coupled with the corresponding inner conductors 3 by external coupling capacitances created between the input/output electrodes 5 and the inner conductors 3.
This dielectric filter is formed of two stages of resonators formed respectively by the resonator holes 2a and 2b. The resonators are coupled in a comb-line manner by the stray capacitances created at the open ends by the non-formation sections 3a formed in the vicinity of the open end surfaces. The thus-constructed dielectric filter, having the resonators coupled with each other by non-formation sections 3a, requires no coupling means such as a coupling hole formed between resonator holes 2a and 2b to couple the resonators in TEM mode, and therefore has the advantage of being capable of being reduced in size.
Ordinarily, this kind of dielectric filter uses a resonant frequency in TEM mode as a fundamental frequency. However, resonance in TE mode, for example, occurs in addition to the resonance in TEN mode. A response at a resonance frequency in this response is an unnecessary mode and is spurious in dielectric filters using TEM mode.
FIG. 9 shows a frequency-attenuation characteristic of a dielectric filter of the above-described construction using a dielectric block having a size of 5 mm along the direction of arrangement of the inner conductors, 4 mm along the lengthwise direction of the inner conductors and 2 mm along the direction of thickness perpendicular to the former two directions, and having a dielectric constant of 92.
As shown in FIG. 9, above a TEM mode fundamental frequency of 1.9 GHz, TE101 mode exists at 5 GHz, TE102 mode at 7.4 GHz, TE201 mode at 8.4 GHz, and TE103 mode at 10.2 GHz. The amount of attenuation at the fundamental frequency of TEM mode is 1 dB while the amount of attenuation of each TE mode is 20 dB. The frequency positions and the amounts of attenuation of these TE modes may be such that the amount of attenuation at double or triple the frequency of TEM mode which is a fundamental mode is smaller, for example 30 bB smaller. In such a case, there is a possibility of failure to achieve a required characteristic (specified value). There is a need to improve the corresponding spurious response characteristic.
In the above-described conventional dielectric filter, however, resonance frequencies (spurious frequencies) of TE mode or the like are determined substantially definitely according to the shape of the dielectric block. Therefore, it has been attempted to change the external size of the dielectric block in order to obtain a required spurious response characteristic. That is, in order to improve the spurious response level (amount of attenuation) at a predetermined (necessary) frequency with respect to each of the different required characteristics to control a spurious frequency of TE mode or the like, the width, thickness and length of the dielectric block are changed according to the required characteristic so that the spurious response is shifted to a higher or lower frequency. In manufacturing the conventional dielectric filter, therefore, there is a need for preparing a multiplicity of dielectric blocks having various shapes for the purpose of improving spurious response characteristics of TE mode or the like according to required characteristics. For this reason, it is difficult to adapt a common or standard dielectric block to the conventional dielectric filter. Therefore, the productivity of the dielectric filter is reduced, the manufacturing cost of the dielectric filter is increased and it is difficult to standardize mounts for the filter.